The invention relates to the field of switching power converters and amplifiers. More particularly, the invention relates to switching power converters and amplifiers having a low power mode for conserving power.
In a conventional switching-mode power converter, a current from a power source, such as an unregulated supply, passes through an inductor when a main power switch is closed. This charges the inductor with energy. When the switch is opened, the energy is discharged into a capacitor. This forms an output voltage across the capacitor which may then be used for driving a load. This output voltage is maintained at a constant, desired level by appropriately controlling the opening and closing of the switch, such as by pulse-width modulation (PWM) or frequency modulation.
In accordance with PWM, the duty-cycle for opening and closing the main power switch is controlled. For example, a periodic ramp signal is compared to a variable signal to control the duty cycle of the switch. A level of power delivered to the load depends upon a duty cycle of the main power switch.
In accordance with frequency modulation, the frequency at which the main power switch is opened and closed is controlled. For example, a voltage-controlled oscillator (VCO) can be utilized for controlling the main power switch. A level of power delivered to the load depends upon the switching frequency.
A conventional switching power amplifier is a type of switching power converter in which the output is varied in response to an input signal. This is in contrast to a switching power converter in which the output is typically maintained at a constant level. In a typical switching power amplifier, the opening and closing of the main power switch is controlled in response to a level of the input signal. Therefore, the output of the switching amplifier tends to follow the input signal.
A class D audio amplifier is an example of a conventional type of switching power amplifier which provides a differential output signal across a speaker using an H-bridge arrangement of four main power switches. Each pair of the switches is coupled serially between a high potential of a power supply and a low potential. Nodes intermediate to each pair of power switches are connected to opposite terminals of the speaker. By controlling the opening and closing of the four main power switches in accordance with an input audio signal, the audio speaker generates sounds related to the input signal. Because the output signal is differential, such an amplifier is typically operated in open loop (i.e. without feedback). A drawback to open loop operation of such an amplifier is that the output signal is susceptible to distortion.
Conventional switching power converters tend to provide efficiency advantages over other types of devices. This is because switching power converters tend to draw power at a rate commensurate with the requirements of the load and tend to have relatively low energy losses. Energy loss in a switching power converter is typically referred to a switching loss and is largely due to energy dissipation in the main power switch(es) and in the reactive elements, such as inductors or capacitors. When a load consumes a level of power which is near the maximum capacity of the switching power converter, the switching losses are low in comparison to the total power drawn by the switching power converter. Thus, the converter operates with high efficiency. As the level of power consumed by the load decreases, however, switching losses become more significant in comparison to the level of power drawn from the power supply. Thus, at low levels of load power consumption, switching losses can significantly reduce efficiency. Efficiency, however, is often an important performance criteria for a switching power converter. For example, where the power is drawn from a battery supply, such as in a portable telephone, low efficiency results in reduced periods of operation between battery charges. Therefore, conventional switching power converters tend make inefficient use of battery power when a load draws a low level of power.
Therefore, what is needed is a switching power converter which does not suffer from the aforementioned drawbacks. It is to these ends that the present invention is directed.
The present invention is a low power mode and feedback arrangement for a switching power converter. Two or more main power switches, such as transistors, transfer energy from a supply to a load by their opening and closing. When the load requires a relatively low power level, this condition is detected. In response, one or more of the transistor switches is disabled from switching and the reduced power requirements of the load are handled by the remaining one or more transistor switches. As a result, switching losses are reduced. This is because parasitic gate capacitance and on-resistance associated with the disabled switches no longer consume power from the power source. The invention provides significant efficiency advantages during periods when the load draws a low level of power. This is especially useful for battery-powered devices which may operate in a low power mode for extended periods of time, such as standby mode as in a portable telephone. The prevention of power loss may contribute considerably to operating time before battery re-charging is needed. However, when additional power is required, one or more previously disabled switches may be brought back into operation to ensure that the power requirements of the load are met.
In an audio amplifier having a H-bridge arrangement for providing a differential output signal, the invention also provides a feedback path from only one side of the load. This reduces distortion associated with open-ended operation.
In accordance with one aspect of the present invention, a switching power converter is provided having first and second transistor switches. Energy is transferred from a supply to a load by opening and closing the first transistor switch; energy is transferred from the supply to the load by opening and closing the second transistor switch; and the second transistor is disabled from switching upon detection of a low power condition.
The first switch may transfer energy from the supply to a first reactive element upon closing and the first switch may transfer energy from the first reactive element to a second reactive element upon opening. The second transistor switch may transfer energy from the supply to a third reactive element upon closing and the second transistor switch may transfer energy from the third reactive element to the second reactive element upon opening. Switching of the first and second transistor switches may be interleaved. The first and second transistor switches may be coupled in parallel to each other. Switching of the first transistor switch may be synchronized with switching of the second transistor switch when the second transistor switch is actively switching. The first transistor switch may be disabled from switching when the second transistor switch is actively switching. The switching power converter may include a pulse-width modulation switch controller coupled to the first and second switches. The switching power converter may include a frequency modulation switch controller coupled to the first and second switches. The low power detector may detect the low power condition by monitoring a voltage provided to the load or by monitoring an error signal representative of a difference between a voltage provided to the load and a desired level for the voltage. The low power detector may include a comparator having a hysteretic transfer characteristic. The first and second transistor switches may have substantially different current-carrying capacities. Energy may be transferred to the load by a current which is synchronously rectified. The switching power converter may also include a third transistor switch and a fourth transistor switch, the third and fourth transistor switches for performing synchronous rectification on the current, wherein the fourth transistor is disabled by the low power detector upon detection of the low power condition.
In accordance with a further aspect of the invention, a switching power converter is provided which includes a switch controller and a plurality of transistor switches including a first transistor switch and a group of at least two additional transistor switches. Each transistor switch is coupled to the switch controller for transferring power from a supply to a load by the switch controller opening and closing the corresponding transistor switch. A low power detector is coupled to the group of at least two additional transistor switches for selectively disabling switching of one or more of the transistor switches of the group in accordance with a detected level of power provided to the load.
The plurality of transistor switches may be coupled in parallel to each other. The transistor switches of the plurality that are actively switching may be synchronized. The switch controller may be a pulse-width modulation switch controller. The switch controller may be a frequency modulation switch controller. The low power detector may detect the level of power provided to the load by monitoring a voltage provided to the load or by monitoring an error signal representative of a difference between a voltage provided to the load and a desired level for the voltage. The low power detector may include a comparator having a hysteretic transfer characteristic.
In accordance with another aspect of the invention, a switching amplifier is provided for receiving an input signal and for generating and output signal representative of the input signal. A first pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a reactive element upon switching of the first pair of transistor switches; a second pair of transistor switches alternately raises and lowers a voltage potential of the first terminal of the reactive element upon switching of the second pair of transistor switches; a switch controller controls switching of the first and second pairs of transistor switches in accordance with the input signal for forming the output signal at a second terminal of the reactive element. A low signal level detector is coupled to the second pair of transistor switches for disabling the second pair of transistor switches from switching upon detection of a low input signal level condition.
The input signal may be an audio signal. The switching amplifier may include a speaker coupled to receive output signal. The switching amplifier may include one or more additional pairs of transistor switches for alternately raising and lowering a voltage potential of the first terminal of the reactive element, each additional pair being selectively disabled by the low signal level detector upon detection of a corresponding low signal level condition (FIG. 9B). The transistor switches of the first pair may have substantially equal current carrying capacities which are substantially different from current carrying capacities of the transistor switches of the second pair. The low signal level detector may detect the low power condition by monitoring the input signal or by monitoring an error signal representative of a difference between the input signal and a reference level. The low power detector may include a window comparator. The window comparator may have a hysteretic transfer characteristic. The switching amplifier may include a feedback path from a terminal of the reactive element to the switch controller for limiting the output signal. The first pair of transistor switches may be actively switching when the low signal level condition is not detected. The first pair of transistor switches may be disabled by the low signal level detector when the low signal level condition is not detected.
In accordance with yet another aspect of the invention, a switching amplifier is provided for receiving an input signal and for generating a differential output signal representative of the input signal. A first pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a first reactive element upon switching of the first pair of transistor switches; a second pair of transistor switches alternately raises and lowers a voltage potential of the first terminal of the first reactive element upon switching of the second pair of transistor switches; a third pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a second reactive element upon switching of the third pair of transistor switches; and a fourth pair of transistor switches alternately raises and lowers a voltage potential of the first terminal of the second reactive element upon switching of the fourth pair of transistor switches. A switch controller controls switching of the first, second, third and fourth pairs of transistor switches in accordance with the input signal for forming a differential output signal across a second terminal of the first reactive element and a second terminal of the second reactive element. A low signal level detector is coupled to the second and fourth pair of transistor switches for disabling the second and fourth pair of transistor switches from switching upon detection of a low input signal level condition.
The input signal may be an audio signal. The switching amplifier may include a speaker coupled to receive the differential output signal. The switching amplifier may include one or more additional pairs of transistor switches for alternately raising and lowering a voltage potential of the first terminal of the reactive element, and one or more additional pairs of transistor switches for alternately raising and lowering a voltage potential of the first terminal of the second reactive element, each additional pair being selectively disabled by the low signal level detector upon detection of a corresponding low signal level condition. The transistor switches of the first pair may have substantially equal current carrying capacities which are substantially different from current carrying capacities of the transistor switches of the second pair. The low signal level detector may be detected the low signal level condition by monitoring the input signal or by monitoring an error signal representative of a difference between the input signal and a reference level. The low power detector may include a window comparator. The window comparator may have a hysteretic transfer characteristic. The switching amplifier may include a feedback path from a terminal of the first reactive element to the switch controller for limiting the differential output signal. A corresponding feedback path from the second reactive element to the switch controller may not be present. The first and third pairs of transistor switches may be actively switching when the low input signal level condition is not detected. The first and third pairs of transistor switches may be disabled by the low signal level detector when the low input signal level condition is not detected. The switching amplifier may be implemented in an eight pin integrated circuit package.
In accordance with a further aspect of the present invention, a switching amplifier is provided for receiving an input signal and for generating a differential output signal representative of the input signal. A first pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a first reactive element upon switching of the first pair of transistor switches; a second pair of transistor switches alternately raises and lowers a voltage potential of a first terminal of a second reactive element upon switching of the second pair of transistor switches; and a switch controller controls switching of the first and second pairs of transistor switches in accordance with the input signal for forming a differential output signal across a second terminal of the first reactive element and a second terminal of the second reactive element. A feedback path is provided from a terminal of the first reactive element to the switch controller for limiting the differential output signal. A corresponding feedback path from the second reactive element to the switch controller may not be present.